Natural Killer (NK) cell activity against YAC-1 lymphoma cells is greatly reduced in spleens of mice injected previously with the bone-seeking isotope, ⁸⁹Sr. Spleen cells from ⁸⁹Sr-treated mice filtered over nylon wool and Sephadex G-10 columns failed to increase the low NK cell activity. Cell mixture experiments failed to show inhibition of NK activity of normal spleen cells by spleen cells from ⁸⁹Sr-treated mice. ⁸⁹Sr produced lowering of NK activity in athymic nude mice similar to that seen in control mice. These experiments suggest that the low NK activity in ⁸⁹Sr-treated mice is not due to the presence of thymus-dependent or independent suppressor cells active at the effector phase of NK cell-mediated killing. The low NK activity of ⁸⁹Sr-treated mice could not be stimulated by administration of interferon inducers, polyinosinic:polycytidylic acid and Corynebacterium parvum, or by preparations containing type I or II interferons. The frequency of cells capable of binding to YAC-1 target cells in normal in spleens of ⁸⁹Sr-treated mice. These data suggest that in ⁸⁹Sr-treated mice, NK cells exist in a state in which they can bind the target cells but cannot lyse them and are unresponsive to the activating effect of interferons. Transformation of these cells into fully lytic and interferon-responsive cells appears to require an intact bone marrow. Bone marrow cell infusions failed to restore NK activity in irradiated or unirradiated ⁸⁹Sr-treated mice, but were able to do so in normal lethally irradiated mice. Thus functional NK cells reactive against YAC-1 lymphoma cells are not only marrow derived but also marrow dependent.